Cast iron material

ABSTRACT

The invention relates to a iron material with graphite flakes, which in a simple manner allows the respective optimal properties to be adjusted for a wide product range by varying the contents of the respective alloy components. For this purpose, cast iron material according to the invention comprises (in % by weight) 3.4 to 4.1% C, 0.9 to 1.4% Si, 0.4 to 0.7% Mn, 0.4 to 0.6% Cu, 0.01 to 0.04% S, 0.003 to 0.007% O 2 , ≦0.04% P and iron and unavoidable impurities as the remainder. In addition, the following may optionally be included singly or in combination 0.15 to 0.45% Mo, 0.005 to 0.02% La, 0.0005 to 0.01% Sr, 0.05 to 0.8% Ni, 0.005 to 0.1% V, 0.05 to 0.15% Sn, 0.05 to 0.08% N and 0.01 to 0.02% Ce. In this case 0.85%≦S c ≦1.05% applies to the degree of saturation Sc=C %/4.26−0.3*(Si %+P %), and 1.97%≦MEG≦2.07% applies to the respective quantity % MEG=2.25%−0.2 Si %.

The invention relates to a cast iron material with graphite flakes,which is particularly suitable for producing brake discs, lightweightand heavyweight engine blocks and cylinder heads.

Cast iron with graphite flakes (grey cast iron) is a preferredconstruction material owing to its good machinability and veryadvantageous casting properties combined with a low risk of theoccurrence of concealed defects. Blocks for internal combustion enginesare therefore typically cast from cast iron materials of the type inquestion.

The requirements already made nowadays on the tensile strength of thematerial have reached the limits of problem-free applicability ofconventional grey cast iron, however. This is due to the fact that, onthe one hand, increased capacities, for example in the casting ofinternal combustion engines, are demanded and, on the other hand, thatlightweight construction is a central aim of modern cast constructions.Making the situation more difficult is the fact that users are demandingnot only higher tensile strength, of as a rule more than 300 MPa, butalso the optimisation of other properties, such as high thermalconductivity, high resistance to thermo-mechanical fatigue and highresistance to frictional wear and sliding abrasion. The quality of thecasting result is also subject to strict tests.

The requirements with respect to high tensile strengths can basically besatisfied by reducing the carbon and silicon contents or the degree ofsaturation and by alloying Cr, Cu, Ni, Mn or Mo up to a total content ofthe elements added by alloying of about 2%. The resistance tothermo-mechanical fatigue may also be set sufficiently high in this way.

However, said measures lead to a considerable reduction in the castingcharacteristic and the self feeding capacity of the processed cast ironmaterial. The risk of concealed defects and partially carbidicsolidification (edge hardness) occurring increases. At the same time themachinability of the material is considerably impaired. In industrialproduction reject rates of up to 30% must therefore be accepted for theincrease in tensile strength and resistance to thermo-mechanical fatigueattained with said measures.

The demand for high thermal conductivity cannot, however, be met byreducing the carbon and silicon contents or the degree of saturation, oralloying with specific alloy elements since the thermal conductivity ofgrey cast iron is a function of the quantity of graphite contained inthe casting, as is well known, and decreases as the quantities ofgraphite become smaller. The elements added by alloying also basicallylead to a reduction in thermal conductivity.

The latter is particularly noticeable if efficient brake discs are to becast from an appropriately alloyed material having relatively highstrengths.

Owing to the segregation behaviour of these elements, alloying withcarbide-forming elements, such as Cr and Mo, leads to the formation ofundesirable complex carbides even if it takes place within theoreticallimits for the solubility of these elements (rule of thumb: atomicradius of the respective element <1.15× atomic radius Fe). Apart fromthe fact that these carbides are “waste products” with adverse effectson the machinability, this has the fundamental drawback that if castmaterial that is produced in the casting operation is re-used inrecirculation, the entropy of the entire system of the circuit isincreased.

When recycling the material re-used in recirculation, the carbides inparticular are, as a rule, not completely destroyed. Instead they areretained as what are referred to as clusters which form carbides againwhen they solidify. As a consequence of renewed alloying with therespectively stipulated quantities of chromium and molybdenum, newcarbides are formed. As a result, this process of enrichment of theprocessed cast material with carbides leads to a slow, but unavoidable,increase in unusable chromium and molybdenum which ends in a creepingdeterioration in the properties of the cast material. As a consequenceof the fact that the segregated elements affect the temperature of theeutectic equilibrium in the Fe—C—X system in different ways and theinactive non-metallic phases also present in the melt are likewisesubjected to the process of creeping increase, feared “inverse chill”casting defects can occur in the casting operation in extreme cases.

In addition to the above-mentioned prior art, a cast iron material isknown from EP 1 213 071 A2 for producing camshafts which (in % byweight) comprises 3.5 to 3.7% C, 0.9 to 1.1% Si, up to 1% Mn andlanthanum that is not bound to sulphur at a content of 0.02 to 0.05% andmay optionally contain 0.3 to 0.6% Cr, 0.1 to 1.0% Cu, 0.3 to 0.6% Moand 0.02 to 0.05% Ti. Lanthanum is added to the known material with theaim of increasing the hardness of the material and bringing about grainrefining that improves the tribological behaviour. The properties of analloy with this composition have been discussed in detail in EP 1 213071 A2 using an embodiment which comprised (in % by weight) 3.69% C,0.95% Si, 0.05% La, 0.029% S, 0.0035% O, 0.29% Mn, 0.5% Cr, 0.2% Cu,0.51% Mo and 0.022% Ti.

A further example of a cast iron material with graphite flakes is knownfrom EP 1 004 789 A1. This material is used for producing brake discswhich are distinguished by increased service life. For this purpose, thecast material known from EP 1 004 789 A1 contains, in % by weight, 3.9to 4.2% C, 0.7 to 1.2% Si, up to 0.02% P, up to 0.02% S and up to 0.05%Al. The known material may also have Mn, V, Cu and Cr contents, whereinthe total fraction of these alloy elements should not exceed 1.6%. Abrake disc produced from a material of this type is distinguished byparticularly high thermal conductivity combined with good toughness. Theknown alloy has been specifically tested with the aid of an embodimentwhich contained (in % by weight) 4.1% C, 1.0% Si, 0.02% P, 0.03% S, 0.3%Mn, 0.01% V, 0.4% Cu, 0.3% Mo and 0.015% Al.

Starting from the above described prior art, the object of the inventionwas to create an alloy concept which easily allows the respectiveoptimum properties to be adjusted for a wide product range by varyingthe contents of the respective alloy components.

This object is achieved according to the invention by a cast ironmaterial with graphite flakes, with the following composition (in % byweight):

C: 3.4 to 4.1%,

Si: 0.9 to 1.4%,

Mn: 0.4 to 0.7%,

Cu: 0.4 to 0.6%,

S: 0.01 to 0.04%,

O₂: 0.003 to 0.007%,

P: ≦0.04%,

with the remainder comprising Fe and unavoidable impurities,

wherein the composition can also optionally contain one or more of thefollowing elements:

Mo: 0.15 to 0.45%,

La: 0.005 to 0.02%,

Sr: 0.0005 to 0.01%,

V: 0.005 to 0.1%,

Ni: 0.05 to 0.8%,

Sn: 0.05 to 0.15%,

N: 0.05 to 0.08%

and 0.85%≦S_(c)≦1.05% applies to the degree of saturation Sc=C%/4.26−0.3*(Si %+P %) (C %: respective C content, Si %: respective Sicontent, P %: respective P content), and 1.97%≦MEG≦2.07% applies to therespective quantity % MEG=2.25%−0.2 Si % (Si %: respective Si content).

The invention provides an Fe—C—Si—X cast alloy which, in particular, hasa combination of properties which is optimised both with respect to itsstrength and with respect to its thermal conductivity and pourabilityand in which the risk of a creeping decrease in the good propertiesoccurring in the practical casting operation is reduced to a minimum.

Cast iron material according to the invention is largely free fromundesirable or unnecessary elements and by-products. Thus the sulphurand oxygen contents are of such a size that they no longer have adisruptive influence on the properties of the iron material. As aresult, the iron lattice is purified and contains sufficient freecapacity for absorbing necessary foreign atoms. Minimum oxygen andsulphur contents are also stipulated as the two elements serve asbuilding blocks for the formation of crystal nuclei.

By adhering to the adjustment rules stipulated according to theinvention for the degree of saturation and the quantity of eutecticgraphite, the carbon and silicon contents are such that, even with acomparatively wide variation in the degree of saturation S_(c), theeutectic quantity of graphite MEG is high.

The quantity of eutectic graphite MEG present in the cast materialaccording to the invention exceeds considerably that of normal castiron. The MEG value thereof is typically only about 1.85% by weight. Inthe cast material according to the invention a volume fraction that ishigher by 10% to 20% is thus available. A decisive advantage of the castiron material according to the invention compared with conventional ironmaterial lies in this excess. Thus material according to the inventionhas a far superior self feeding capacity for the purpose of balancingout the shrinkage in the iron with expansion of the graphite, comparedwith conventional cast material. In the casting operation in practicethis property leads to a clear increase in the reliability with whichhigh quality cast products are produced.

When producing a cast material according to the invention the reducingmelt treatment by seeding should be strongly oriented toward therespective level of the oxygen and/or sulphur contents.

As alloy elements the invention provide elements of which the atomicradius does not differ too greatly from that of iron. The difference ispreferably up to a maximum of 2%. The alloy elements should not bestrong carbide formers and should not segregate directly. According tothe invention it is therefore provided that, if required, copper,nickel, manganese or molybdenum are added by alloying to the ironmaterial in order to adjust its respectively required properties. Tin,of which the atomic radius is up to 50% greater than that of iron, mayalso be added for this purpose.

Accordingly cast iron material according to the invention containscopper in quantities of 0.4% by weight to 0.6% by weight in order topromote the formation of pearlite without adverse effects on the desiredhigh degree of graphitisation. A further positive effect of the presenceof Cu lies in the fact that segregation directions are formed on thiselement. When producing lighter cast parts, such as lightweight engineblocks, it has proven to be advantageous if the range of Cu contents islimited to 0.45 to 0.55 in order to achieve these effects.

By way of addition the alloy according to the invention may also containnickel in amounts of 0.05 to 0.8% by weight, preferably 0.05 to 0.7% byweight. Nitrogen contents of 0.05 to 0.08% by weight can also beprovided in combination with Ni or on their own. The two alloy elementsensure that high strengths are obtained in the finished cast part evenin the event of partial pearlite breakdown. Ni and N are thereforepresent in the iron material according to the invention in combinationor singly, preferably in particular if cast parts are produced which,owing to their shaping or mass, cool slowly with the danger of thepearlite breaking down. The rule in this case should be that the Niand/or N contents are higher, the greater the modulus of the respectivecast part.

The technical term “modulus” in this case designates the ratio of castpart volume to heat-emitting area, for which “cm” is usually used as theunit of measurement.

Mn contents in the range of 0.4% by weight to 0.7% by weight likewiseassist the formation of pearlite. Manganese is added in particular,however, to form segregation directions on manganese. To produce lightercast parts that cool more quickly, the Mn contents can be limited to therange of 0.45 to 0.65% by weight to achieve this effect.

The maximum phosphorus content is limited to 0.04% by weight to minimisethe formation of phosphite eutecticum which would be harmful to thetoughness of the material. The sulphur content should also be limited toa maximum of 0.04% by weight to avoid sulphide formations for thisreason. If Cer is present the contents, provided according to theinvention, of at least 0.1% by weight are used for nucleation whichleads to superfinely distributed oxysulphides. The following rule can beapplied: if Ce is present, the Ce content should be adjusted so as to behigher, the greater the respective S content. The oxysulphides formed byCer in conjunction with sulphur promote the formation of graphite andbring about an increase in the strength and hardness of the materialwithout reducing the toughness thereof.

Mo can be added to the cast iron material according to the invention inamounts of 0.15% by weight to 0.45% by weight to block displacementmovements by diffusion from the iron lattice in the event of thermalstress and consequently to prevent the introduction of cracking. Thereliability with which the properties, established by the addition ofMo, of the material according to the invention are achieved may beincreased in that the upper limit of the Mo content is restricted to0.35% by weight and the lower limit raised to 0.2% by weightrespectively.

Tin contents, which are 0.05% by weight to 0.15% by weight, lead, withlonger residence time of the cast part in the mould, to the formation ofa micro segregation zone around the graphite flakes and preventdiffusion of carbon from the graphite and into the basic matrix.

Addition of strontium promotes nucleation and development of a structurethat is advantageous with respect to the desired properties. At least0.0005% by weight Sr are required to reliably achieve this purpose. Apositive effect can no longer be ascertained with contents of more than0.01% by weight on the other hand. In particular in the case of largercast parts in which the strength is particularly important, aparticularly positive effect is established if Sr is present in amountsof 0.0005 to 0.002% by weight.

Lanthanum contents in the range of 0.005 to 0.02% by weight have afavourable effect on the pourability of the cast alloy according to theinvention and promote the hardness of the material and its tribologicalbehaviour by inducing grain refining.

If necessary vanadium is added to the alloy according to the inventionin order to increase the hardness and tensile strength of the material.Vanadium alloys the cementite of the pearlite and leads to the formationof shorter, rounded flakes in the graphite flakes with the result thatthe hardness and toughness increase. If vanadium is added to an alloyaccording to the invention for this purpose, it can be done as afunction of the modulus of the respective component in order to reliablyattain the desired degree of success. The V content should increase withincreasing thickness in this case. Thus practical tests have shown thatoptimum cast part properties are established if with a modulus of therespective cast part of 0.25 to 0.65 cm the V content is 0.025 to 0.035%by weight, with a modulus of 0.65 to 1.2 cm the V content is >0.035% to0.065% by weight and with a modulus larger than 1.2 cm the V content ismore than 0.055 to 0.1% by weight. The solubility limit is exceeded withcontents according to the invention of more than 0.1% by weight.

A variant of the alloy according to the invention that is particularlysuitable for producing brake discs is characterised in that its carboncontents are in the range of 3.8 to 4.1% by weight. The relatively highcarbon content leads to strengths which are in the range of 150 to 200MPa. At the same time cast parts produced from the alloys with this typeof composition have high thermal conductivity combined with a high levelof toughness. The silicon content is preferably in the range of 0.9 to1.2% by weight for the same purpose.

For casting cast parts in which a high strength combined with goodthermal conductivity is to the fore, a further variant of the inventionprovides that the C content is in the range of 3.4 to 3.8% by weight, inparticular 3.4 to 3.6% by weight.

Tests have shown that cast iron material according to the invention witha composition of this type has high strengths which are regularly morethan 300 MPa in the cast state.

When casting thick-walled cast parts, it is also advantageous if the Sicontent of the alloy is 1.15 to 1.4% by weight, in particular 1.2 to1.4% by weight, in order to meet the danger during casting ofre-oxidation with reduced C contents.

The oxygen contents of a cast iron alloy according to the invention haveparticular significance. Speed and extent of nucleation are controlledby the O₂ content. Thus an increase in the oxygen content leads to rapidparticle growth, while lower oxygen contents result in less growth.These effects are achieved with O₂ contents which are in the range of 30to 70 ppm. If brake discs or similarly configured components areproduced from the alloy according to the invention, optimum structuresmay be obtained via the oxygen content such that the oxygen contents arelimited to 30 to 40 ppm. With thin-walled cast parts, such as lightengine blocks or the like, with a modulus of 0.1 to 0.4 cm, high O₂contents of 50 to 70 ppm have proven to be advantageous as they promotefast grain growth within the respective short cooling time. In the caseof thick-walled components with moduli in the range of 0.4 to 1 cm, forexample heavy engine blocks, optimised structural properties areachieved if the O₂ content is 40 to 60 ppm. When casting cast parts witha complex shape, such as cylinder heads, with a modulus in the range of1 to 2.5 cm, on the other hand, grain growth that is optimised withrespect to the properties demanded of these components is achieved ifthe O₂ content of the alloy according to the invention is in the rangeof 30 to 50 ppm.

The high tensile strengths of a cast material according to the inventionmay be particularly reliably ensured in that in the cast state more than50% of the oxygen contained in the cast iron material according to theinvention is in the form of a type of oxide of which the startingtemperature of the reduction with oxygen is above 1,700 K.

In addition to the improved strength, thermal conductivity, toughnessand machinability cast iron material according to the invention also hasgood corrosion resistance. As a result of this specific combination ofproperties, cast iron material according to the invention isparticularly suitable for producing brake discs and engine blocks orcylinder heads for internal combustion engines. In particular the hightensile strengths combined with good pourability, machinability and highthermal conductivity make the material according to the inventionparticularly suitable for use as material for producing blocks formodern diesel engines, in which extremely high pressure loads occur inthe region of the combustion chamber during the course of the combustionprocess.

The properties of cast iron material according to the invention havebeen proven in a large number of examples.

Thus HGV brake discs have been cast from cast iron alloys according tothe invention with the compositions B1 to B7 given in Table 1a in % byweight, the Sc value, % MEG value, tensile strength Rm and Brinellhardness HB of which are given in Table 1b. Table 1b also contains anevaluation of the structure of the products obtained in each case.

It has been found that the HGV brake discs cast from the alloys given inTable 1a have tensile strengths in the region of 160 to 230 MPa. Thehardness values are in the range of 147 to 220 in this case, so thebrake discs have good wear resistance in addition to high strengths.They also have outstanding thermal conductivity, so they can reliablyabsorb and discharge the forces acting on them even in the case of highloads.

Table 2a gives the contents of C, Si, S, Mn, Cu, V, Mo, Sn and Ni foralloys D1 to D5 of cast iron materials according to the invention, fromwhich thin-walled car engine blocks with a modulus of 0.7 to 0.8 cm havebeen cast. The relevant alloys D1 to D6 also contained 60 ppm by weightO₂ and 0.01% by weight La in each case. Table 2b contains the associatedvalues % MEG, SC, tensile strength Rm and Brinell hardness HB averagedover various measuring points in each case, as well as an evaluation ofthe structure.

Table 3a gives the contents of C, Si, S, Mn, Cu, V, Mo, Sn and Ni foralloys Z1 to Z6 of cast iron materials according to the invention, fromwhich cylinder heads weighing 100 kg (alloys Z1 to Z4) and 400 kg(alloys Z5, Z6) have been cast. The modulus of the 100 kg cylinder headswere between 2.5 and 3 cm while the modulus of the cylinder headsweighing 400 kg was 1 cm. The relevant alloys Z1 to Z6 also contained 40ppm by weight O₂ and 0.01% by weight La. Table 3b contains theassociated values % MEG and SC, tensile strength Rm and Brinell hardnessHB and an evaluation of the structure.

Finally, a heavy crankcase was cast from a cast iron alloy according tothe invention consisting of (in % by weight) 3.6% C, 1.35% Si, 0.1% Sn,0.5% Mn, 0.5% Cu, 0.01% V, 0.2% Mo, 40 ppm by weight O₂ and 0.03% S,with the remainder being iron and unavoidable impurities. The SC valueof the alloy was 0.93 and its % MEG value was 1.98. The finished casehad a tensile strength Rm of 320 MPa and a finely structured pearliticconstruction.

The invention thus provides a cast iron material which has a superiorproperty spectrum which can be varied over a wide range. The materialaccording to the invention is characterised by particularly goodmachinability. Its high tensile strength allows known castconstructions, which previously had been produced only from conventionalgrey cast iron, to be produced with higher strengths without expensiverestructuring being necessary. TABLE 1a C Si S Mn Cu V Mo O₂ La Sn B13.90 0.97 0.050 0.48 0.52 0.091 0.43 0.0035 0.01 B2 3.96 1.02 0.028 0.520.51 0.088 0.47 0.0035 0.01 B3 3.95 1.14 0.037 0.53 0.54 0.093 0.400.0035 0.01 B4 4.07 1.08 0.043 0.48 0.48 0.083 0.02 0.0035 0.01 B5 4.091.22 0.027 0.47 0.50 0.098 0.41 0.0035 0.01 B6 3.99 0.95 0.020 0.52 0.560.006 0.27 0.0035 0.01 B7 3.94 1.05 0.026 0.55 0.60 0.005 0.29 0.00350.01 0.10Remainder iron and unavoidable impurities

TABLE 1b SC % MEG Rm HB Structure B1 0.98 2.056 191.0 207 Finelystructured pearlite, Carbide < 1% B2 1.00 2.046 207.3 210 Finelystructured pearlite B3 1.01 2.022 231.2 201 Finely structured pearlite,Carbide < 1% B4 1.03 2.034 172.1 186 Finely structured pearlite B5 1.052.006 143.9 171 Finely structured pearlite, Centre ferrite B6 1.00 2.060162.5 147 Finely structured pearlite B7 1.00 2.040 185.2 173 Finelystructured pearlite, Carbide < 1%

TABLE 2a C Si S Mn Cu V Mo Sn Ni D1 3.67 1.24 0.026 0.57 0.69 0.005 0.300.09 0.63 D2 3.60 1.20 0.031 0.48 0.50 0.044 0.19 0.01 D3 3.60 1.210.026 0.48 0.49 0.044 0.19 0.01 D4 3.48 1.22 0.038 0.48 0.51 0.045 0.190.01 D5 3.52 1.34 0.037 0.52 0.46 0.043 0.20 0.01Remainder iron and unavoidable impurities

TABLE 2b SC % MEG Rm HB Structure D1 0.943 2.002 224.9 222 Finelystructured pearlite D2 0.923 2.010 252 222 Finely structured pearlite D30.923 2.008 244.7 185 Finely structured pearlite D4 0.893 2.006 290.4189 Finely structured pearlite D5 0.912 1.982 277.3 201 Finelystructured pearlite

TABLE 3a C Si S Mn Cu V Mo Sn Ni Z1 3.45 1.16 0.023 0.53 0.60 0.045 0.34Z2 3.56 1.18 0.010 0.51 0.66 0.005 0.19 Z3 3.53 1.09 0.028 0.54 0.520.036 0.29 Z4 3.57 1.23 0.024 0.60 0.56 0.045 0.29 0.10 Z5 3.51 1.190.027 0.57 0.59 0.039 0.30 Z6 3.38 1.12 0.025 0.57 0.68 0.049 0.31 0.65Remainder iron and unavoidable impurities

TABLE 3b SC % MEG Rm HB Structure Z1 2.018 0.88 350.0 229 Finelystructured pearlite Z2 2.014 0.91 237.1 186 Finely structured pearliteZ3 2.032 0.90 210.0 187 Finely structured pearlite Z4 2.004 0.92 281.5215 Finely structured pearlite Z5 2.012 0.90 295.3 195 Finely structuredpearlite Z6 2.026 0.86 313.2 205 Finely structured pearlite

1. Cast iron material with graphite flakes, with the followingcomposition (in % by weight): C: 3.4 to 4.1%, Si: 0.9 to 1.4%, Mn: 0.4to 0.7%, Cu: 0.4 to 0.6%, S: 0.01 to 0.04%, O₂: 0.003 to 0.007%, P:<0.04%, the remainder comprising Fe and unavoidable impurities, whereinthe composition may also optionally contain one or more of the followingelements: Mo: 0.15 to 0.45%, La: 0.005 to 0.02%, Sr: 0.0005 to 0.01%,Ni: 0.05 to 0.8%, V: 0.005 to 0.1%, Sn: 0.05 to 0.15%, N: 0.05 to 0.08%Ce: 0.01 to 0.02% and 0.85%≦S_(c)≦1.05% applies to the degree ofsaturation Sc=C %/4.26−0.3*(Si %+P %) (C %: respective C content, Si %:respective Si content, P %: respective P content), and 1.97%≦MEG≦2.07%applies to the respective quantity % MEG=2.25%−0.2 Si % (Si %:respective Si content).
 2. Cast iron material according to claim 1,characterised in that the C content is 3.8 to 4.1% by weight.
 3. Castiron material according to claim 2, characterised in that the Si contentis 0.9 to 1.2% by weight.
 4. Cast iron material according to eitherclaim 2 or claim 3, characterised in that the O₂ content is 0.003 to0.004% by weight.
 5. Cast iron material according to claim 1,characterised in that the C content is 3.4 to 3.6% by weight.
 6. Castiron material according to claim 5, characterised in that the Si contentis 1.15 to 1.4% by weight.
 7. Cast iron material according to eitherclaim 5 or claim 6, characterised in that the Sr content is 0.005 to0.002% by weight.
 8. Cast iron material according to any one of claims 5to 7, characterised in that the V content is 0.025 to 0.045% by weight.9. Cast iron material according to any one of claims 5 to 8,characterised in that the Sn content is 0.05 to 0.15% by weight. 10.Cast iron material according to any one of claims 5 to 9, characterisedin that the Si content is 1.15 to 1.25% by weight.
 11. Cast ironmaterial according to any one of claims 5 to 10, characterised in thatthe O₂ content is 0.003 to 0.005% by weight.
 12. Cast iron materialaccording to any one of claims 5 to 10, characterised in that the O₂content is 0.004 to 0.006% by weight.
 13. Cast iron material accordingto any one of claims 5 to 10, characterised in that the O₂ content is0.005 to 0.007% by weight.
 14. Cast iron material according to any oneof the preceding claims, characterised in that the S content is at least0.02% by weight.
 15. Cast iron material according to any one of thepreceding claims, characterised in that the Mo content is 0.2 to 0.4% byweight.
 16. Cast iron material according to any one of the precedingclaims, characterised in that the Mn content is 0.45 to 0.65% by weight.17. Cast iron material according to any one of the preceding claims,characterised in that the Cu content is 0.45 to 0.55% by weight. 18.Cast iron material according to any one of the preceding claims,characterised in that its Sr content is at least 0.05% by weight. 19.Cast iron material according to any one of the preceding claims,characterised in that in the cast state more than 50% of the oxygencontained therein is in the form of a type of oxide of which thestarting temperature of the reduction with oxygen is above 1,700 K.